"A collection of insults 12" joke

As sharp as a sack full of wet mice. -- Foghorn Leghorn

As smart as a politician/lawyer is honest.

As smart as bait / an automatic email responder script.

As smart as Christie Brinkley is ugly.

As thick as champ. (Irish; champ is mostly mashed spuds and cabbage.)

As thick as two short planks / two half bricks.

As worn out as a cucumber in a convent.

Attic's a little dusty.

Back burners not fully operating.

Bad spot on the disk.

Baler done run out of twine.

Bandwidth limited.

Barney's his hero.

Bats have flown the belfry, and now he's all alone.

Bats in the belfry.